samples: bpf: force IPv4 in ping

ping localhost may default of IPv6 on modern systems, but
samples are trying to only parse IPv4.  Force IPv4.

samples/bpf/tracex1_user.c doesn't interpret the packet so
we don't care which IP version will be used there.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

samples/bpf/sockex1_user.c

@@ -26,7 +26,7 @@ int main(int ac, char **argv)
 	assert(setsockopt(sock, SOL_SOCKET, SO_ATTACH_BPF, prog_fd,
 			  sizeof(prog_fd[0])) == 0);
 
-	f = popen("ping -c5 localhost", "r");
+	f = popen("ping -4 -c5 localhost", "r");
 	(void) f;
 
 	for (i = 0; i < 5; i++) {